in debian/rules we have the line below which appears to set the version for the package. However this currently sets the version to 1.5 even on the 1.4 branch.
override_dh_gencontrol: dh_gencontrol -- -v$(shell (git describe --tags --long --match 'vyos/*' --dirty 2>/dev/null || echo 0.0-no.git.tag) | sed -E 's%vyos/%%' | sed -E 's%-dirty%+dirty%')
kylem@management:~/git/vyos-1x# git branch current equuleus * sagitta kylem@management:~/git/vyos-1x# git describe --tags --long --match 'vyos/*' --dirty 2>/dev/null vyos/1.5dev0-23-gd08c61286